Active stress field and fault kinematics of the Greater Caucasus

Journal Pre-proofs Active stress field and fault kinematics of the Greater Caucasus A. Tibaldi, N. Tsereteli, O. Varazanashvili, G. Babayev, A. Barth, T. Mumladze, F.L. Bonali, E. Russo, F. Kadirov, G. Yetirmishli, S. Kazimova PII: DOI: Reference:

S1367-9120(19)30460-2 https://doi.org/10.1016/j.jseaes.2019.104108 JAES 104108

To appear in:

Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

12 June 2019 21 October 2019 23 October 2019

Please cite this article as: Tibaldi, A., Tsereteli, N., Varazanashvili, O., Babayev, G., Barth, A., Mumladze, T., Bonali, F.L., Russo, E., Kadirov, F., Yetirmishli, G., Kazimova, S., Active stress field and fault kinematics of the Greater Caucasus, Journal of Asian Earth Sciences (2019), doi: https://doi.org/10.1016/j.jseaes.2019.104108

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Active stress field and fault kinematics of the Greater Caucasus A. Tibaldi1)*, N. Tsereteli2), O. Varazanashvili2), G. Babayev3), A. Barth4), T. Mumladze2), F.L. Bonali1), E. Russo1), F. Kadirov3), G. Yetirmishli5), S. Kazimova5) 1) University

of Milan Bicocca, Milan, Italy M. Nodia Institute of Geophysics, M. Javakhishvili Tbilisi State University, Tbilisi, Georgia 3) Geology and Geophysics Institute, Azerbaijan National Academy of Sciences, Baku, Azerbaijan 4) Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany 5) Republic Center of Seismic Service, Azerbaijan National Academy of Science, Baku, Azerbaijan 2)

*Corresponding author: Prof. Alessandro Tibaldi, [email protected]

Abstract This work contributes to depict the current seismicity, fault kinematics, and state of stress in the Greater Caucasus (territories of Georgia, Azerbaijan and Russia). We merged and homogenized data from different earthquake catalogues, relocated ~1000 seismic events, created a database of 366 selected focal mechanism solutions, 239 of which are new, and performed a formal stress inversion. Preferential alignments of crustal earthquake foci indicate that most seismic areas are located along the southern margin of the belt and in the north-eastern sector. This is consistent with the presence of dominant active WNW-ESE faults, parallel to the mountain range. In the entire Greater Caucasus, a dominant NNE-SSW-oriented greatest principal stress (1) controls the over-all occurrence of earthquakes of minor and major magnitude. Main earthquakes are characterized by a vertical least principal stress (3), corresponding to reverse kinematics. Reverse slip is more common along the southwestern and north-eastern foothills of the Greater Caucasus, although in these areas there are also scattered strike-slip events. This suggests the presence of local stress fields with horizontal 1 and 3. In the central-southern part of the mountain belt, in correspondence of the local collision between the Lesser and the Greater Caucasus, 1 rotates to NNW-SSE. The strike-slip events, instead, dominate along the southern flank of the central-eastern mountain range; this is interpreted as the effect of the collision that promotes eastward escape of the tectonic blocks located to the east.

1

Key words: Caucasus, stress field, seismicity, active fault, focal mechanisms 1 Introduction The Caucasus is the main orogenic belt of central Asia resulting from the Cenozoic collision between the Arabian and the Eurasian plates (Sosson et al., 2016). The convergence caused the development of two main fold-and-thrust belts, known as the Greater Caucasus (GC) and the Lesser Caucasus, separated by the Transcaucasian depression for most of their lengths (Fig. 1) (Adamia et al., 1977, 2010; Banks et al., 1997; Mosar et al., 2010; Sosson et al., 2010). The cumulated post-collisional sub-horizontal shortening of the Caucasus caused by the northward motions of the African-Arabian plates is estimated at hundreds of kilometers (Barrier and Vrielynk, 2008; Meijers et al., 2013). Based on apatite fission-track data, the exhumation process in the GC started in the Oligocene and reached its highest rate in the Miocene-Pliocene (Avdeev, 2011; Avdeev and Niemi, 2011; Vincent et al., 2007, 2011). GPS and seismic data indicate that the convergence between the Eurasian and the African-Arabian plates is ongoing (Reilinger et al., 2006; Kadirov et al., 2008, 2012; Avagyan et al., 2010; Adamia et al., 2017; Alizadeh et al., 2016; Sokhadze et al., 2018). As a result, the GC and the Lesser Caucasus are still tectonically active, with vertical and horizontal components of motions that imply ongoing mountain building processes (Rebai et al., 1993; Jackson and Ambraseys, 1997; Koçyiğit et al., 2001; Allen et al., 2004; Reilinger et al., 2006; Tan and Tayemaz, 2006; Pasquaré et al., 2011; Kadirov et al., 2015; Tibaldi et al., 2017a, b). In the Georgian part of the GC, field data indicate that recent shortening has occurred along major structures striking WNW-ESE (Tsereteli et al., 2016, and references therein). Focal mechanism solutions (FMS) provide evidence for dominant thrust faulting along planes striking parallel to the mountain belt, thus consistent with field data (Jackson and McKenzie, 1984; Tsereteli et al., 2016). Fault planes mostly dip to the north in the western GC, whereas in the central-eastern GC, thrust faults located at the two mountain sides (northern and southern) have converging dips (Philip et al., 1989; Tan and Taymaz, 2006) (Fig. 1). According to some authors, this tectonic setting is complicated by the effect of the escape of crustal blocks to the west toward the Black Sea and to the east toward the South Caspian basin (e.g. Kazmin, 2002).

2

The tectonically active regions of Caucasus and Transcaucasus host a large concentration of inhabitants, posing a substantial local seismic hazard. Two major urban centers are located in the basin at the southern foothill of the GC: Tbilisi, the capital of the Republic of Georgia with ~1.2 million people, and Baku, the capital of the Republic of Azerbaijan, with ~2.3 million inhabitants (Fig. 1). Hundreds of rural villages are also scattered in the Transcaucasian region and in the interior of the mountain belt. The great majority of resident houses or public edifices, such as hospitals, are not built according to anti-seismic regulations. Some of these villages are also home to historical edifices with a high architectural and touristic relevance, such as Mestia in Georgia, which is included in the list of the UNESCO World Heritage Sites. To add to the potential geohazards, this region hosts fundamental international energy infrastructures and lifelines: among them the Baku-Supsa and the Baku-Tbilisi-Ceyhan oil pipelines, the South Caucasian natural gas pipeline, and the Enguri hydroelectric facility. The latter is located at the foothills of the western GC and furnishes 40% of energy to the Republic of Georgia (European Bank, 2018) and 100% of energy to the separatist territory of Abkhazia (Fig. 1). The high level of urbanization together with the presence of key economic infrastructures contribute to greatly increase the seismic hazard and risk associated with the Caucasus and Transcaucasus regions. The Anatolia-Caucasus region has been also characterized by intense and continuous volcanic activity, at least from the Jurassic until Present (e.g. Rebai et al., 1993). The magmatism in the central-western part of the Caucasus mountain belt, which is associated with the collision between the Eurasian and the Arabian plates, started in Late Miocene (Adamia et al., 1977, 2008; Koronovskii and Demina, 1999) and in more recent times gave rise to large volcanoes, most of which are still active, such as Elbrus, Chegem, Keli and Kazbegi (Adamia et al., 2011b). The presence of several volcanoes on top of an active compressional belt is a puzzling and still unsolved question (e.g. Tibaldi, 2008), since pure contractional tectonics is usually considered as a highly unfavourable setting for volcanism (Cas and Wright, 1987; Glazner, 1991; Hamilton, 1995; Watanabe et al., 1999). Similarly, there are several mud volcanoes in Azerbaijan under compression (Babayev et al., 2014). In spite of the societal and hazard relevance of these regions, the location, geometry and kinematics of the seismogenetic structures are poorly documented. This is in part due to the isolation of these regions during the Soviet era, the logistical

3

difficulties with extreme rugged terrains and very few roads, and the fact that most of the tectonic compression occurs by substantial aseismic deformation, as suggested by a shortening rate that is a factor of 3 to 10 higher than it can be accounted for by solely considering seismicity (Jackson and McKenzie, 1988; Philip et al., 1989; Jackson and Ambraseys, 1997; Reilinger et al., 1997; McClusky et al., 2000). Modern analyses have been recently carried out to link crustal seismicity with fault structures identified both by seismic reflection surveys and field geological-structural data (Tsereteli et al., 2016). However, these studies have been conducted only in the territory of the Republic of Georgia and integration with similar data from the conterminous Azerbaijan and Russian regions is fundamental. The present paper seeks to contribute towards the integration and harmonization of seismic and field geological structural data acquired by different countries to further our scientific knowledge in the assessment of the seismic hazard of the GC and Transcaucasian regions. We compiled an updated and complete catalogue of seismicity across the Georgia-Russia-Azerbaijan territories of the CaucasianTranscaucasian regions, based on shared common procedures and magnitude scale. We then relocated 1000 earthquakes and crated a database of 366 selected FMS, 239 of which were calculated here. Finally, we calculated the modern stress field for the studied regions. In this way, we wish not only to share fundamental data useful for seismic hazard/risk assessment under a trans-boundaries perspective, but also to contribute to the following challenging scientific questions: i) Is the variation in fault dip along the GC reflected in the state of stress? ii) Is there consistency between the stress field in the Caucasus region and the plate motion vectors? iii) Can the regional stress state, or its local variations, explain the presence of Holocene/active volcanoes, such as Mt Elbrus, in a compressional setting? iv) Is the stress field consistent with previous hypothesis on lateral escape of parts of the Caucasus belt?

4

Figure 1. Location of the Greater Caucasus and main active faults (faults after Tan and Taymaz, 2006; Forte et al., 2014; Tsereteli et al., 2016). Triangles are on the hanging-wall block of reverse faults. Fault planes mostly dip to the north in the western Greater Caucasus, whereas in the central-eastern Greater Caucasus, thrust faults located at the two mountain sides (northern and southern) have converging dips. The tip line trace of the Dagestan Blind Thrust (DT) is projected to the surface. MCT = Main Caucasus Thrust. 2 Geological setting We describe here the main general features of the Arabian-European collision zone from north to south. The GC is characterized by a series of Mesozoic-Cenozoic rocks piled up along ramp-and-flat fault planes, following the Alpine inversion of the previous extensional tectonic regime (Adamia et al., 1977, 2010, 2011a; Gamkrelidze, 1986; Phillip et al., 1989; Shikhalibeyli, 1996; Banks et al., 1997; Saintot et al., 2006; Mosar et al., 2010; Sosson et al., 2010, 2013; Forte et al., 2014). Part of the pre-Alpine basins survived in the form of the present-day Caspian and Black Sea depressions. In the Transcaucasian region, the Rioni and Kura developed as foreland basins (Adamia et al., 2011a; Forte et al., 2010, 2014; Mosar et al., 2010; Alania et al., 2016) (Fig. 2). South of the Transcaucasian region, the Lesser Caucasus fold-and-thrust belt is separated from the Pontide accreted terrains by the ophiolitic Sevan-Akera suture zone. The axial zone of the GC is characterized by the presence of pre-Mesozoic basement and Jurassic sedimentary rocks, whereas laterally consist mainly of

5

Cretaceous to Cenozoic sedimentary rocks (Adamia et al., 2011a; Mosar et al., 2010). These rocks are affected by WNW-ESE-striking reverse faults with quasirectilinear surface trace parallel to the mountain belt. Two main structural salients can be identified. The first one is located in the Rioni Basin where part of the rock succession has been uplifted, mainly since the Miocene, giving rise to an arcuate fold-and-thrust belt with the convex side pointing southward (Tibaldi et al., 2017a, 2017b) (Fig. 1). In the north-eastern GC, an arcuate blind fault (Sobornov, 1994), known as Dagestan Thrust (Fig. 1), or also as North Caucasus Thrust (Kadirov et al., 2008), dips southward and allows propagation of shortening further northward than in the remaining northern Caucasus front, creating the second and larger salient (Forte et al., 2014). WNW-ESE-striking main faults affect the inner part of the GC fold-and thrust-belt and separate it from the Scythian Platform (SP) to the north (Fig. 2). The SP is a physiographic platform representing the southern margin of the European plate. It was characterized, during the Jurassic-Eocene, by subsidence, followed by collision tectonics in the Oligocene-Neogene-Quaternary (Adamia et al. 2011a,b). In particular, during the middle Miocene-Quaternary, the rock succession of the southern SP was involved in five main contractional phases, directly related to the convergence between the European and the Arabian plates, with the youngest phase lasting from 1.6 Ma ago to the Present (Nikishin et al., 1998). Each compressional phase was accompanied by folding, reverse faulting and rapid subsidence of molasse basins. The combination of the various events of subsidence of different origin, produced a minimum of 5 km of Palaeozoic to Quaternary sedimentary deposits that cover the basement of the SP, giving rise in particular in the studied region, to the Terek-Kaspian foredeep basin (Saintot et al., 2006). In the GC fold-andthrust belt instead, metamorphic and intrusive rocks are much shallower. The southern folded and faulted border of the SP and the less deformed deposits of the Terek-Kaspian foredeep have been unified in one main tectonic unit in Figure 2. The central core of the GC corresponds to the main fold-and-thrust belt, characterized by intense deformation and the largest presence of recent and active faults (Fig. 2). The Transcaucasus region, as already mentioned, comprises, from west to east, the Rioni basin, the Georgian block and the Kura basin. The Rioni foreland basin is triangular-shaped, open towards the Black Sea basin, with a flat morphology interrupted by the Rioni salient. The Georgian block is a high-topography zones that

6

interrupts the continuity of the depressed Rioni and Kura basins. The block is made of uplifted Mesozoic-Cenozoic deposits and older crystalline rocks belonging to the Dzirula Massif (Mayringer et al., 2011; Okrostsvaridze and Tormey, 2013). The Kura foredeep basin is a wide depressed area that extends up to the Caspian Sea coast, interrupted by slightly uplifted areas made of folded and faulted basin deposits. The Kura basin unit represented in Figure 2 includes also the Alazani basin. In regards to the current deformation, the convergence rate between the Arabian plate and the Eurasian plate is ca. 20-30 mm/yr, according to geodetic data (De Mets et al., 1990, 1994; Reilinger et al., 2006). This rate decreases towards the western Caucasus (Reilinger et al., 2006). GPS stations along the northern Arabian plate move at a rate of 18 ± 2 mm/yr towards N25° ± 5°W relative to Eurasia. The GPS data indicates that total convergence across the central and western Lesser Caucasus and GC is ~10 ± 2 mm/yr (McClusky et al., 2000). Stations located in the Transcaucasian depression move at 6 ± 2 mm/yr towards NE in a Eurasia-fixed reference frame, suggesting that ~60% of the shortening occurs within the GC (McClusky et al., 2000). Repeating GPS measurements in Azerbaijan in 1998-2018 define active convergence between the Lesser Caucasus/Kura Depression and the GC with strain concentrated along the Main Caucasus Thrust (Fig. 1) (Philip et al., 1989; Reilinger et al., 2006; Kadirov et al., 2008, 2012; Telesca et al., 2013, 2017). Present day slip rates on the Main Caucasus Thrust decrease from 10±1 mm/yr in eastern Azerbaijan to 4±1 mm/yr in western Azerbaijan (Kadirov et al., 2008, 2015).

7

Figure 2. Map of largest historical and instrumental earthquakes (Ms > 5.5, Mw > 5.7), plotted above the main tectonic units (tectonic units simplified after Adamia et al., 2011a). 3 Seismicity of the Greater Caucasus 3.1 Data Earthquake data are obtained from the integration of several catalogues including the New Catalogue of Strong Earthquakes in the USSR from Ancient Times through 1977 (Kondorskaya and Shebalin, 1982), the Catalogue of Large Historical Earthquakes of the Caucasus (Shebalin and Tatevossian, 1997), the Catalogue of Historical earthquakes in Georgia (up to 1900) (Varazanashvili et al., 2011), the Recent Developments of the Middle East Catalogue (Zare et al., 2014), Earthquakes of Azerbaijan for 1983–2002 (Gasanov, 2003), and the Catalogue for GC (Mumladze et al., 2016). We integrated our earthquake data with the catalogues updated for the period 2006-2017 for Georgia by Varazanashvili et al. (2018) and for Azerbaijan by Telesca et al. (2012, 2017). The catalogue for the northern Caucasus in the Russian territory was based on data from the International Seismological Center (ISC). The depths of the earthquakes in the GC area are not well constrained. Current uncertainties in the earthquake locations are both due to large errors in the phasearrival times of seismic waves, and to the usage of a 1D velocity model across the entire region investigated. Recently, most of the research efforts of the Caucasian countries have been focused on the recalculation of earthquake hypocenters to decrease the current uncertainties (e.g. Tutberidze et al., 2004; Tsereteli et al. 2012, 2016; Mumladze et al., 2015; Adamia et al., 2017; Kazimova et al., 2017). In the present paper, we relocated 450 independent events for Georgia and 550 for Azerbaijan with MS > 2.8. To obtain a reliable estimation of hypocenters, we used polarity estimations, take-off angle estimations, station locations and a velocity model. We evaluated existing velocity models based on explosion data: we chose the velocity model with the least scatter between depth and epicenter locations calculated for explosions and for our data. For a complete description of the method we refer to Tutberidze et al. (2004) and Kazimova et. al. (2017). Travel times were also picked directly from seismograms or from bulletins. 3.2 Magnitude harmonization between earthquake catalogues Unification of earthquake catalogues is essential for characterization of seismicity. In the southern Caucasus, until 2003, earthquake magnitude was

8

described by the energy class (K) (Rautian, 1964), and the regional magnitude MPV. The latter is determined using the vertical component of P-wave oscillations on seismic records as seen on short-period sensors in the field with Δ ≤ 300 km (sometimes, Mb  MPV was also assumed). For earthquakes with MS > 3.5, this parameter was estimated directly by using its regional calibration curve. Following Rautian and Khalturin (1978), it was accepted that MS  MC, where MC is the magnitude estimated from coda waves. The empirical correlation formula between (K) and MS, and between MPV and MS for the considered region (Rautian, 1964) was used to unify the catalogue by MS up to 2004. Starting in 2004, because of the changes in

the Georgian and Azerbaijan

seismic networks, often only local magnitudes (ML) were calculated for recorded earthquakes. In this case, the conversion from ML to Ms magnitude was carried out through the relationship provided by Kalafat et al. (2009), whereas conversion from ML to Mw through the relationship by Zahre et al. (2014). In the present study, we derived an empirical relationship between ML and Kvalues for Georgia. In Georgia, old analogue seismic stations and new digital stations were concurrently operational for one year only in 2004; thus the K-values are calculated from January 2004 until December 2004. The analogue data to calculate K-values are based on two seismic stations in Georgia (TI2, AKH) and twelve seismic stations from neighbouring countries (Fig. 3A). For the same time period, ML was calculated using five digital stations. ML and K-values were calculated for a total of 567 earthquakes (Fig. 3B).

9

Figure 3. (A) Number of K-values at seismic stations in Georgia (TI2, AKH) and Azerbaijan (all others). (B) Relationship between local magnitude ML and K-value. The resulting relation is: ML = 0.5494K - 1.9308

(1)

Unfortunately, Ms estimations are not available for this period for Georgian earthquakes, however using the relationship between Ms and K for the Caucasus region by Rautian (1964): K = 1.8 Ms + 4

(2)

the following relations between ML and Ms can be obtained: ML = 0.9889Ms + 0.2668

(3)

Ms = 1.011ML - 0.2698

(4)

10

A comparison of our results obtained by equations (3) and (4) with those obtained by Kalafat et al. (2009) yielded similar results (Fig. 4). Average standard deviation for equations (3) and (4) is 0.2, which is comparable to the estimated error for Ms. In Azerbaijan analogue and digital seismic networks have been concurrently operational from 2003 to 2011. After the Caspian Earthquake sequence on 25 November 2000 which included two mainshocks of Mw 6.08 and 6.18 respectively, (Babayev et al., 2014), Azerbaijan significantly contributed to the modernization of the seismic network. From 2003 to 2011, thirty-five kinemetrics instruments were installed and equipped with telemetric (satellite) communication channels. The additional data allowed investigating the correlation between magnitudes determined from analogue equipment (MPV, MLH, K) and digital stations (ml, mb, MPSP, MS) (Yetirmishli et al., 2015).

8 7

ML

6 5 4

ML-Geo

3

ML-Tur

2

ML-Azer

1 0 0

1

2

3

4 Ms

5

6

Figure 4. Correlation between ML and Ms for Turkey, Azerbaijan and Georgia. Evaluating the regional correlation formula for Mw and ML for Georgia was not feasible due to the limited amount of Mw values. As a consequence, for the harmonization of catalogues to Mw, we have used the equation of Zare et al. (2014) for Georgia and the regional equation of Yetirmishli et al. (2015) for Azerbaijan. 3.3 Recurrence of earthquakes and magnitudes The recurrence law of earthquakes, that can be determined from the magnitude-frequency distribution of Gutenberg-Richter, is one of the indicators of the current seismicity of a given region. Main parameters involved in this distribution are:

11

i) a-value, which indicates the seismic activity in terms of spatial and temporal occurrences within a certain period; this parameter is thus associated with the level of seismic activity of a particular area, and ii) b-value, which determines the ratio of the number of large earthquakes compared to smaller ones; it is calculated using cumulative distribution of magnitude frequency curves for independent events (based on de-clustered catalogues, i.e. mainshock without aftershocks and foreshocks). The values of parameters a and b depend on the length of the temporal and spatial windows considered. We applied different de-clustering algorithms (e.g. Reasenberg, 1985; Grünthal et al., 1999; Uhrhammer, 1986; Gardner and Knopo, 1974) to our updated Georgian catalogue. Results varied based on the method used. We obtained 754 independent events with the Reasenberg method, 1433 events with the Uhrhammer method, 674 events with the Grünthal method, and 929 events with the Gardner and Knopo method. Differences originated for the smaller earthquakes in the range of MS 2.8-3.8. Average standard deviation of b-value for these estimations is 0.2. Finally, we chose the method by Gardner and Knopoff (1974) because the b-value is averaged respect to the other methods. Figure 5 shows the epicentre distribution for independent events of Caucasus and adjacent regions between 1900 and 2017. The catalogue for the territory of Turkey near the Georgia border was provided by National Earthquake Monitoring Center of Bogazici University (Turkey). The Catalogue for the territory of Iran was provided by the International Institute of Earthquake Engineering and Seismology (IIEES) (Shahvar et al., 2013; Karimiparidari et al., 2013). The Catalogue of Armenia was taken from Sargsyan et al. (2017).

12

Figure 5. Map of independent earthquake epicenters of Caucasus region and main active faults, triangles on hangingwall block of reverse faults. The two lines A-B and C-D locate sections of Figure 14. Due to the development of the seismological network in the Caucasus, the representativeness of the data also changed. Here and in the following sections, we analysed the earthquake magnitude of completeness of the catalogue, the earthquake spatial distribution, the depth distribution and FMS separately for the SP and for the remaining GC region. This distinction has been adopted based on the different characteristics of the SP that should be reflected in the actual possible different seismic behaviour of the core and southern side of the GC, with respect to the SP. The main distinctive characteristics of the SP are: i) a different history and pre-collisional/syn-collisional evolution, ii) a thicker cover of sedimentary deposits, and iii) the location between the highly deformed inner GC and the more stable European platform. The magnitude (MS) of completeness is equal to 4.5 for the period of observation 1900-1961 in the GC and in the SP (Figs. 6A and 6C), whereas for the period 1962-2016, from the graph the magnitude of completeness is 2.5 in the GC and SP (Figs. 6B and 6D). However, since this value for earthquakes with MS < 3.5 has been estimated by correlation equation between k and MS, we have also to consider that the error is higher than for earthquakes with larger magnitudes. Taking

13

into account these uncertainties, we are more inclined to give a more conservative value of magnitude of completeness of 3.0 for GC and SP during 1962-2016.

Figure 6. Investigation of magnitude by annual frequency distribution (expressed in logarithmic scale) for the Greater Caucasus during 1900-1961 (A) and 1962-2016 (B), and for the Scythian platform during 1900-1961 (C) and 1962-2016 (D). A regression analysis was used to study magnitude-frequency distribution separately for the fold-and-thrust belt of the GC and the SP, resulting in a different recurrence law for the GC fold-and-thrust belt (eq. 5) and for the SP (eq. 6): logN = 3.97 - 0.85 MS

(5)

logN = 3.87- 0.90 MS

(6)

Our results show that the seismic activity of the fold-and-thrust belt of the GC is double the seismic activity in the SP at the level corresponding to Ms = 3.5. The recurrence period of earthquakes with Ms = 7 in the fold-and-thrust belt of the GC is about 100 years, whereas the recurrence period for the same earthquake magnitude in the SP is about 300 years. 3.4 Spatial distribution of the largest earthquakes

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In this section, we focus on the largest earthquakes with Ms > 5.5 (Mw > 5.7) (Fig. 2, Table 1) included in the merged/updated catalogue. The spatial distribution of these main earthquakes is not uniform along the GC. Although the westernmost part of the mountain belt is seismically less active, two large events occurred in the area near the Novorossiysk town in historical times known as the Nizhnyaya-Kuban earthquakes of 150 BC (Ms = 6.1) and 1879 AD (Ms = 6.0). East of these epicentres, there is a gap in large seismic events, which reappears more to the east with Ms of 67. These events are concentrated along the southern side and foothills of the GC. In the central part of the GC, in correspondence with the active Kazbek volcano (Fig. 2), large earthquakes increase in number and they are dispersed across the southern and northern side of the mountain range. Along the northeastern foothills, the epicentres of the largest events are located mostly towards the foreland. The oldest large earthquake is the Kvira event of 1250 BC (Ms = 6.9), followed by other six main events with Ms ranging from 6 to 7. The eastern part of the mountain range is the most seismically active and includes the largest events described in the entire Caucasus, spanning in age from 742 AD to 2012 AD, with Ms ranging from 5.7 to 7.8. In particular, the Shamakhi area has been characterised by tens of large earthquakes from 1192 AD to 1902 AD, and Ms in the range 5.7-7.8. From 1902 AD to 2019 AD, the Shamakhi area was a comparatively seismically-quiescent zone, until February 2019 when an event with Mw = 5.5 occurred. Elevated seismic activity is also observed in the southern margin of the SP, the Terek-Caspian Foredeep and the Dagestan area. Here several events occurred from 650 AD to 1976 AD, with Ms in the range 5.7-6.8. The active compressional belt extends down into the Caspian Sea, this is confirmed by the occurrence of 7 earthquakes (from 1902 AD to 2000 AD) with Ms = 5.6–6.8. Table 1. List of the largest earthquakes (Ms > 5.5) Westernmost part Yr

M

D

H

Min

Sec

Lat

Long

Dep

MS

MW

IO

Local name

Sources

150 BC

00

00

00

00

00.0

44.60

38.10

14

6.1

6.1

8.5

Nizhnyay a Kuban' 1

1600

00

00

00

00

00.0

43.40

41.00

15

7.0

7.0

9.5

Bzipi

1750

00

00

00

00

00.0

42.90

41.90

15

7.0

7.0

9.5

Akiba

1879

10

09

19

30

00.0

45.10

37.90

25

6.0

6.1

7.0

Nizhnyay a Kuban' 2

Shebalin, Tatevosian, 1997 Varazanashvil i et al., 2011 Varazanashvil i et al., 2011 Shebalin, Tatevosian,

15

1997

1891

00

00

00

00

00.0

43.05

41.30

15

6.0

6.1

8.0

Amtkel

1905

10

21

11

01

26.0

43.30

41.70

35

6.4

6.4

7.0

Teberda

1963

07

16

18

27

14.0

43.18

41.65

10

6.4

6.4

9.0

Chkhalta

Varazanashvil i et al., 2011 Kondorskaya and Shebalin, 1982 Shebalin, Tatevosian, 1997

Central part 1250 BC 1100

00

00

00

00

00.0

42.70

42.20

015

6.9

6.9

09.5

Kvira

00

00

00

00

00.0

43.10

42.30

015

7.0

7.0

09.5

1350

00

00

00

00

00.0

42.70

43.10

015

7.0

7.0

09.5

1750

00

00

00

00

00.0

43.00

42.70

015

6.9

6.9

09.5

1921

06

29

11

37

55.0

43.80

42.80

025

6.0

6.1

06.0

NenskraA bakura Lechkhu miSvaneti Labaskal di-Tseri Kislovods k

1991

06

15

00

59

19.0

42.38

43.98

017

6.1

6.1

08.0

Java

2009

09

07

22

41

35.8

42.57

43.48

020

6.0

6.3

07.5

Oni 2

Varazanashvil i et al., 2011 Varazanashvil i et al., 2011 Varazanashvil i et al., 2011 Varazanashvil i et al., 2011 Kondorskaya and Shebalin, 1982 Shebalin, Tatevosian, 1997 Zare et al., 2014

Eastern part 0742

00

00

00

00

00.0

42.40

44.90 018

6.4

6.4

08.5

Jvari pass

1192

07

00

00

00

00.0

40.70

48.60 010

6.1

6.1

09.0

Shamakhi 1

1250

00

00

00

00

00.0

41.60

47.20 015

5.7

5.9

07.5

Ikhrek

1530

00

00

00

00

00.0

42.05

45.40 015

5.7

5.9

08.0

Alaverdi 1

1668

01

14

00

00

00.0

41.00

48.00 040

7.8

7.7

10.0

Shamakhi 2

1668

01

21

00

00

00.0

41.50

47.00 015

6.6

6.6

09.0

Shamakhi

1668

02

07

00

00

00.0

41.00

48.00 030

6.8

6.8

08.0

Shamakhi

1668

05

07

00

00

00.0

41.00

48.00 030

6.8

6.8

08.0

Shamakhi

1668

07

29

00

00

00.0

41.00

48.00 030

7.1

7.1

08.5

Shamakhi

1671

01

11

12

00

00.0

41.50

48.70 015

6.2

6.2

08.5

Shamakhi

1742

08

05

00

00

00.0

42.10

45.60 020

7.0

7.0

09.0

Alaverdi 2

1828

08

09

16

00

00.0

40.70

48.40 010

5.7

5.9

08.0

Shamakhi 3

Varazanash vili et al., 2011 Shebalin, Tatevosian, 1997 Kondorskay a and Shebalin, 1982 Varazanash vili et al., 2011 Shebalin, Tatevosian, 1997 Shebalin, Tatevosian, 1997 Shebalin, Tatevosian, 1997 Shebalin, Tatevosian, 1997 Shebalin, Tatevosian, 1997 Shebalin, Tatevosian, 1997 Varazanash vili et al., 2011 Kondorskay a and Shebalin,

16

1982

1859

06

11

13

00

00.0

40.70

48.50 012

6.1

6.1

08.5

Shamakhi 4

1872

01

28

07

00

00.0

40.60

48.70 011

6.0

6.1

08.5

Shamakhi 5

1902

02

13

09

39

00.0

40.70

48.50 018

6.9

6.9

08.5

Shamakhi 6

1934

10

29

16

15

00.0

40.66

49.01 030

5.9

6.0

06.5

Sabirabad

1948

06

29

16

06

29.0

41.90

46.80 020

6.1

6.1

07.0

Zakatala1

2012

05

07

04

40

25.4

41.50

46.58 009

5.6

5.7

07.0

Zaqatala 2

Shebalin, Tatevosian, 1997 Shebalin, Tatevosian, 1997 Shebalin, Tatevosian, 1997 Kondorskay a and Shebalin, 1982 Shebalin, Tatevosian, 1997 Zare et al., 2014

Southern Margin of Scythian Platform, Terek-Caspian Foredeep, Dagestan 0650

00

00

00

00

00.0

42.60

47.70

020

6.1

6.1

08.0

Dagestan

0918

00

00

00

00

00.0

42.10

48.20

020

6.1

6.1

08.0

Derbent 1

1652

06

08

20

00

00.0

42.10

47.70

010

5.8

5.9

08.5

Derbent 2

1830

03

09

11

22

00.0

43.10

46.70

013

6.8

6.8

08.5

Dagestan

1878

05

04

00

00

00.0

41.60

48.10

020

5.7

5.9

07.0

Ullu-Gital

1886

10

16

08

35

00.0

43.80

45.40

035

6.0

6.1

06.0

Shchedrin

1906

02

20

20

54

00.0

41.50

48.40

075

6.0

6.1

06.0

UstSamur

1909

10

30

17

36

39.0

42.40

48.00

040

5.8

5.9

06.0

Dagestan

1912

08

10

11

07

36.0

43.50

45.10

050

5.7

5.9

08.0

Tersk

1913

03

25

14

03

56.0

41.80

48.30

007

5.9

6.0

07.0

Derbent

1970

05

14

18

12

00.0

43.00

47.09

017

6.6

6.6

08.5

Dagestan

1976

07

28

20

17

00.0

43.17

45.60

016

6.2

6.2

08.5

Chernogor ia

1902

02

21

06

21

01.0

41.80

48.80

018

5.6

5.7

06. 0

Caspian Sea 1

1911

06

07

23

58

48.0

41.00

50.50

052

6.4

6.4

06. 5

Caspian Sea 2

1913

03

25

14

03

56.0

41.80

48.30

007

5.9

6.0

07. 0

Derbent

Kondorskaya and Shebalin, 1982 Kondorskaya and Shebalin, 1982 Kondorskaya and Shebalin, 1982 Shebalin, Tatevosian, 1997 Kondorskaya and Shebalin, 1982 Shebalin, Tatevosian, 1997 Kondorskaya and Shebalin, 1982 Kondorskaya and Shebalin, 1982 Kondorskaya and Shebalin, 1982 Kondorskaya and Shebalin, 1982 Shebalin, Tatevosian, 1997 Shebalin, Tatevosian, 1997

Caspian Sea Kondorskaya and Shebalin, 1982 Shebalin, Tatevosian, 1997 Kondorskaya and Shebalin,

17

1982

1935

04

09

19

59

00.0

42.20

49.00

100

6.3

6.3

06. 0

Caspian Sea 4

1961

09

18

11

01

04.0

41.10

50.20

035

6.0

6.1

07. 0

Caspian Sea 5

1963

01

27

19

35

00.0

41.80

49.84

035

6.2

6.2

07. 5

Caspian Sea 6

2000

11

25

18

09

11.0

40.24

49.95

050

6.8

6.8

07. 0

Baku 1

Shebalin, Tatevosian, 1997 Shebalin, Tatevosian, 1997 Shebalin, Tatevosian, 1997 Zare et al., 2014

3.5 Depths distribution of the largest earthquakes We here consider the depth distribution of the largest earthquakes separately for the fold-thrust belt of the GC (Fig. 7A) and the SP (Fig. 7B). Around 95% of the total earthquakes in the fold-thrust belt of the GC and around 85% in the SP is located in the top 20 km of the crust. In both the fold-thrust belts of the GC and the SP, the largest number of events occur at depths of 5 and 10 km, and in the SP also at 15-25 km depth. In the fold-thrust belt of the GC, 4% of the earthquakes are located between 21 and 40 km, and the remaining 1% is found at depths ranging between 41 and 80 km. In the SP, 12% of the earthquakes are between 21 and 45 km, and the remaining 3% is located at depths greater than 46 km, up to 160 km. Based on a comparison with the distribution of the main rock formations from Sikharulidze et al. (2004), it appears that earthquakes with MS < 5 mostly occurred in sedimentary deposits, and larger earthquakes with MS ≥ 5 mostly took place in granite bodies.

18

Figure 7. Number of earthquakes vs. depth for the Greater Caucasus (A), and for the Scythian Platform. Red bars = Distribution of number of earthquakes with depth; blue line = Trend by moving average by step 2. 3.6 Focal mechanism solutions 3.6.1 Methods Numerous studies in the past have investigated earthquake fault plane solutions for the Caucasus region using a variety of methodologies (Ambraseys and Jackson, 1998; Jackson and McKenzie, 1988; Fuenzalida et al., 1997; Ekstrom and England,1989; Pondrelli et al., 1995; Tan et al., 2006). In Georgia, the most recent investigation of the active stress field and its relationships among the different seismotectonic units was carried out at various scales by Tsereteli et al. (2016), Adamia et al. (2017), and Tibaldi et al. (2018). We provide here improvements to these previous studies by presenting a database of 239 new and 127 selected FMS for 366 major earthquakes that occurred in Georgia and Azerbaijan. For the new FMS of earthquakes occurred before 2003, we used the first motion polarity technique (Lander, 2004). For the post-2003 events of Georgia, we used the frequency sensitive moment-tensor inversion technique (Barth et al., 2007). The

19

newly obtained FMS are integrated with the FMS of Azerbaijan for the period 20032017, provided by Republican Center of Seismic Service (RCSS) of the Azerbaijan National Academy of Sciences (ANAS). Focal mechanism data of 1953-2003 are from Agayeva et al. (2009) and Babayev et al. (2017). The entire data set of 19532017 has been based on the first motion polarity technique (Lander, 2004). The frequency sensitive moment-tensor inversion technique by Barth et al. (2007) was not used for Georgian earthquakes after 2009 due to the fact that waveform data of the local network are not accessible to all Georgian scientists. Table 1s (supplementary documents) presents all data used in the analysis. Only earthquakes for which the number of polarities is equal to or greater than 8 and azimuthal gaps are less than 180° were included in the analysis, which led to a reduction of the dataset to 366 earthquakes. For the Azerbaijan territory, the Azerbaijan Republic Center of Seismic Survey (RCSS) provided 33 FMS, whereas further 206 FMS have been calculated in the present work. For the Georgia territory, the Institute of Geophysics of Tbilisi provided 67 FMS, whereas further 33 FMS have been here calculated. For North Caucasus, other 21 FMS come from ISC and 6 from Balakina et al. (1996). 3.6.2 Results Results show that, in general, 30% of FMS have reverse fault slip along planes striking WNW-ESE, consistent with the dominant orientation of the known recent/active reverse faults (Fig. 2). 34% of FMS has strike-slip kinematics, whereas other focal mechanisms show transpression (6%), transtension (4%) and normal slip (4%) (Fig. 10). The remaining 22% have not clear solutions. In Figures 8A, B and C it is possible to observe that most earthquakes at depth < 10 km have reverse kinematics in the western and central part of the mountain belt, and strike-slip kinematics in the eastern part. Most reverse events have slip planes striking WNWESE, similarly to the events located in the mountain belt. Strike-slip events along the southern border of the GC have, in most cases, one slip plane parallel to the mountain front. At depths of 10-15 km (Fig. 9A), strike-slip events decrease in number, while events with reverse mechanism increase. At these depths, also some FMS with normal motions do occur. Planes of strike-slip earthquakes are more rotated with respect to shallower events, having in most cases the slip planes oblique with respect to the WNW-elongation of the GC. A few strike-slip events have one slip

20

plane parallel to the southern mountain front. Reverse FMS have slip planes striking parallel to the GC elongation.

Figure 8. FMS of the Greater Caucasus region calculated in the present work and integrated with other selected FMS. A. Hypocenter depth < 5 km. B. and C. Hypocenter depth of 6-10 km. Colour of beach balls is referred to depth.

21

Figure 9. FMS of the Greater Caucasus region calculated in the present work and integrated with other selected FMS. A. Hypocenter depth of 11-15 km. B. Hypocenter depth of 16-30 km. C. Hypocenter depth > 30 km. Colour of beach balls is referred to depth.

22

Figure 10. Distribution of the number of earthquakes for all types of fault kinematics for the Greater Caucasus. Un - Unknown; NF - normal fault; NS - normal fault with strike-slip component; SS- strike-slip fault; TS - reverse fault with strike-slip component; TF - reverse/thrust fault. Normal slip planes have different strikes, ranging from parallel to perpendicular to the mountain belt. At depths of 15-30 km, the kinematic and geometric distribution of earthquake mechanisms is similar to the one of 10-15 km, with the exception that normal faulting decreases in frequency. At depths > 30 km, most events are characterized by reverse motions and are concentrated along the eastern part of the GC, with the exception of one event that is located offshore at the foothill of the south-westernmost part of the mountain belt. In the GC, seismic consistency (Cs – a statistic measure showing how similar earthquakes are, Apperson, 1991) is 0.66, and in the SP, Cs is 0.62 (this ratio usually ranges from 0 to 1.0, perfectly consistent). The Cs values obtained for both the GC and the SP thus indicate a medium similarity of fault plane solutions. To define seismically homogeneous areas we subdivided the region into a series of zones that take into account both the spatial earthquake distribution and the regional tectonics (Fig. 11). The subdivision of the available FMS to seismic zones for stress inversion is based on the Earthquake Model of the Middle East (EMME, Danciu et al., 2018) and mostly coincide with the tectonic units previously described (Fig. 2). This model integrates country-specific seismogenic source models and interdisciplinary data to achieve a seismogenic source model based on active tectonics, seismicity and geological evidences. The EMME was built for the purpose of calculating seismic hazard and does not necessarily reflect areas of a consistent stress field. The source

23

regions are partly elongated widely in WNW-ESE direction and thus may represent areas of similar tectonics but include spatially varying stress fields. We found out that for some of those regions, the slip directions of the FMS data could not be explained by a single stress tensor. Thus, to further specify spatial stress variations, we subdivided the original source regions into smaller areas where the FMS data are locally clustered in order to analyse specific stress orientations. The adapted seismic zones include eight regions denoted from A to H. With reference to Figure 12, zone A corresponds to the seismically active Georgian block unit between the Rioni foreland in the West of the study region and the Kura foreland in the East. Zone B covers the seismicity in the western part of the GC fold-and-thrust belt, including the Rioni salient and the metamorphic uplifted complexes. For simplicity, zones A and B are reduced to a rectangular shape to cover the seismicity within; as a consequence, here they do not fully represent the seismotectonic zones. Zones C, E and H represent the Kura basin unit, whereas zones D and F coincide with the central and eastern part of the GC fold-and-thrust belt. The borders between the Kura Basin (C, E, and H) and the central and eastern GC (D and F) are adopted from the borders of the EMME zonation. In contrary to the original source EMME regions, we subdivided the Kura basin into three new zones C, E and H to analyse their local stress fields along the mountain belt GC axis, since the FMS data could not be explained by a single stress tensor. Finally, zone G corresponds to the southern folded and faulted border of the SP and the less deformed deposits of the Terek-Kaspian foredeep. All eight zones have diagnostic characteristics that justify their use for stress inversion because they have suffered different tectonic histories, have distinct morphostructural patterns, and have clustered seismicity (apart from zone G that has a more scattered seismicity). Figure 11 shows the Cs values and distribution of stress regime for each tectonic zone. Zones A and B (Georgian block and Western GC fold-and-thrust belt, respectively) are more homogeneous. In zone B, only thrust faults are present, whereas in zone A is characterized by thrust, transpressional, and strike-slip fault mechanisms. The eastern part of the GC is more complex: in zones C, E, F and H, movements are dominantly of strike-slip type. In zones D and G (Central GC foldand-thrust belt, and SP-Terek-Kaspian foredeep, respectively), thrust faulting is instead the dominant kinematics. In the most complex zone H (Eastern Kura basin), where all types of faulting do occur, Cs is 0.57, lower than in the other studied zones.

24

Figure 11. Cs values and distribution of stress regime for each cluster.

25

Figure 12. Crustal earthquakes divided into eight zones for stress inversion. Most zones correspond to the main tectonic units introduced in Figure 2 and they are based upon Adamia et al. (2011a, b). A: Georgian block, B: Western GC, C: Western Kura basin, D: Central GC, E: Central Kura basin, F: Eastern GC, G: Scythian platform-Terek-Kaspian foredeep, H: Eastern Kura basin. Earthquakes are colourcoded by depth. 3.7 Stress inversion 3.7.1 Methods We performed a formal stress inversion following the method of Michael (1984). This method relies on the assumptions that i) the stress field is uniform and invariant in space and time, and ii) earthquake slip occurs in the direction of the maximum shear stress (Wallace-Bott hypothesis, from Bott, 1959). We included both nodal planes for each FMS to allow finding the best fitting planes during the inversion procedure. This helps for the ambiguity between focal plane and auxiliary nodal plane. Then, we removed the worst fitting nodal plane of each FMS and inverted the data again (Barth and Wenzel, 2010). The inversion algorithm assumes similar shear stress magnitudes on each fault and hence minimises the misfit angle β (the angle between direction of maximum shear stress and the slip direction) and the difference between the normalised shear stress magnitudes |τ| and the reference value |τ0| = 1 (Michael, 1984). Orientations of the horizontal greatest principal stress (Hmax) are

26

computed using equation [insert number] from Lund and Townend (2007). We used all the 366 FMS described above to perform the formal stress inversion. For this purpose, we carried out the stress inversion for each of the homogenous seismotectonic zones described in the previous section (Fig. 12). 3.7.2 Results The magnitudes of the analysed earthquakes span a wide range of values, from 2.2 to 6.9, indicating a high variability of rupture areas. For most of the seismotectonic zones, large slip-angles, up to 20°-45° are observed. This means that the slip vectors of the FMS cannot be explained by a single stress orientation. By reducing the earthquakes within each zone to only large events using an individual lower threshold of the seismic moment M0,min, we achieved stable inversion results (Fig. 13). The thresholds, listed in Table 2, were established for each zone taking into consideration the tectonic and seismological characteristics. A fit angle of 15° or lower was determined for all zones except for the eastern Kura basin (zone H). In combination with the number of inverted FMS, this results in stress orientations of quality A and B according to the World-Stress-Map quality ranking scheme (Barth et al., 2008). The Hmax in all zones, apart from C, is mainly oriented NNE-SSW, more precisely N5°E to N31°E (Fig. 13). In zone C, the Hmax trends N5°W. In the western and eastern parts of the GC, the Hmax trends towards N10-31°, whereas in the central GC, the Hmax trends ca. N-S, from N5°W to N5°E. All regions show dominant compressional tectonics, thus Hmax corresponds to the greatest principal stress (σ1). The regions in the North and Northwest (zones A, B, D and G) show thrust faulting regimes with a subordinate strike-slip component, the horizontal minimum stress is oriented about WNW-ESE and corresponds to the intermediate principal stress (σ2). The southern and eastern parts (C, E, F, and H) reveal mainly strike-slip tectonics with a horizontal least principal stress (σ3). We compared the σ1 orientations obtained from the largest earthquakes, with the entire data set. The σ1 orientations obtained by including all the FMS show similar trends. For zones A, B, C, E, G, and H the deviation is less than 6°, whereas for zones D and F is ~ 20°. These data indicate that our result of the regional stress state is quite stable.

27

Table 2. Stress inversion results for all zones. SS represents strike-slip regimes, TS predominantly thrust faulting with a minor strike-slip component. * indicates only bquality FMS are used. Strong earthquakes

All earthquakes

Region

Zone

σ1 in °N

R

#FMS Fitangle

Quality M0,min

Regi me

σ1 in °N

R

#FMS

Fitangle

Georgian block

A

008.5

0.37

22

10.8°

A

1.7e22

TS

006.0

0.36

39

21.6°

Western GC

B

019.5

0.30

13

08.2°

B

-

TS

see columns on the left

Western Kura basin

C

-04.4

0.46

20

14.8°

B

3.0e23

SS

-04.6

0.60

57

29.5°

Central GC

D

021.4

0.32

15

11.2°

A

-*

TS

002.7

0.38

49

39.4°

Central Kura basin

E

011.9

0.44

10

13.5°

B

5.0e20*

SS

011.1

0.15

33

33.7°

Eastern GC

F

030.7

0.48

16

15.8°

B

3.0e21*

SS

011.2

0.42

83

34.8°

Scynthian G platform and Terek-Kaspian foredeep

003.4

0.48

13

13.3°

B

3.0e21*

TS

003.5

0.15

54

44.5°

Eastern Kura basin

006.6

0.55

10

24.0°

-

5.0e20*

SS

000.6

0.55

33

41.5°

H

28

Figure 13. Formal stress inversion of focal mechanisms. Colours indicate the eight separate inversion zones as indicated in Figure 12. Black beach balls represent earthquakes not used in the inversion. Coloured, converging arrows give the orientations of the greatest principal stress (σ1). Diverging arrows indicate the least principal stress (σ3) for strike-slip regime (black) and the intermediate principal stress (σ2) for predominantly thrust faulting regime (white). Arrow lengths are scaled by stress ratio R. Red triangles indicate Quaternary volcanoes after Lebedeva and Vashakidzeb (2014). 4 Discussion 4.1 Distribution of seismicity Seismicity of the GC is linked to the ongoing convergence between the Eurasian and the Africa-Arabian plates (Nemčok et al., 2011; Kadirov et al., 2012; Alizadeh et al., 2016). Lithospheric shortening gives rise to frequent and large earthquakes that have devastated the Caucasus region (Varazanashvili et al., 2012, Kadirov et al., 2008, 2012, 2015; Babayev, 2009; Babayev et al., 2014; Telesca et al., 2017). Earthquakes are mostly clustered along the GC, following the WNW-ESE trend of the mountain range. In more detail, most of the events are located along the southern and northern foothill regions, whereas a smaller subset of events is located along the GC axis. Moreover, we confirm the general decrease of seismicity in the

29

western regions, as already put forward by other authors (Mumladze et al., 2015, and references therein). In Figure 5, we observe a correlation between the main active faults of the GC and the earthquakes epicentres. Seismicity is concentrated in proximity of main reverse faults, dipping NNE, which border the southern limit of the mountain range. A more diffuse seismicity is possibly linked to a swarm associated with reverse faults that dip SSW in the northeastern part of the GC. These main faults seem to nucleate both low and high magnitude earthquakes, although we suspect that more unrecognized active faults do exist in the mountain belt and, especially, at its foothill regions. We believe that these unrecognized faults, possibly smaller in size, are responsible of the lower magnitude earthquakes, whereas the major events are originated from the main mapped structures. This hypothesis is supported by two main observations: i) detailed field geological-structural and geophysical mapping in the GC have helped to identify new, shorter active faults (Tibaldi et al., 2017a, 2017b, 2019); and ii) there is a well-established relation between surface fault rupture length and earthquake magnitude, such that the longer faults are usually capable of producing the larger earthquakes (e.g. Wells and Coppersmith, 1994; Wyss, 1979; Kim and Sanderson, 2005). It is also possible that some of the unrecognized active structures are blind faults and might have a shallow attitude. Shallow faults have been observed in the southwestern part of the mountain belt, where active and reverse faults are located within the first km of the crust (Tibaldi et al., 2018). This is also confirmed by the depth distribution of seismicity in the GC and in the SP, where clusters of seismicity are observed at depths < 10 km (Fig. 6). A medium size earthquake (MS 4-5) produced by a very shallow fault, can create serious damages of buildings and infrastructures in the epicentral area, especially if construction did not follow anti-seismic regulations. The effects of a shallow, medium size earthquake can even be worst in the case of houses built with poor materials and/or palafitte foundations, as frequently do occur in the rural zones of the Caucasus region. Although depth uncertainties need to be taken into consideration, generally we observed a peak in seismicity at shallow depths, between 5 and 10 km depth (Fig. 7). This is consistent with a typical thin-skinned tectonic model. Thin-skinned tectonics characterizes the Rioni Basin; here, Banks et al. (1997) and Tibaldi et al. (2017a) by means of crustal seismic sections, identified two levels of south-vergent reverse and thrust faults. The upper level thrusts correspond to depths of ~2-5 km, whereas the

30

deeper thrusts detach and flatten along the Upper Jurassic evaporites of the Rioni foreland basin at ~8-10 km depth. It is possible that also other areas have similar upper crust detachments in the sedimentary successions. 4.2 Distribution of earthquake magnitudes and recurrence A comparison between the distribution of the earthquakes and the main geological units shows that in the eastern part of the GC, coinciding with the territory of the Republic of Azerbaijan, all large earthquakes (MS ≥ 5.0) are confined to areas where the crystalline basement crops out (Mammadli, 2012), suggesting a control on seismic foci by the brittle rheology of this rock unit. In the westernmost part of the mountain range, the majority of earthquakes are located in the sedimentary and granite formations. Similarly, clusters of earthquakes are linked to the brittle rheology of the granite units. By comparing the magnitude distributions in relation to the main rock units, we observe that earthquakes with MS < 5 occur mostly in sedimentary deposits, whereas larger earthquakes with MS ≥ 5 take place in granite bodies. This is in agreement with the presence of higher friction along existing slip planes within the intrusive rocks with respect to decollément layers in the sedimentary cover, and to the presence of higher stiffness in granites with respect to sedimentary rocks in the case of new ruptures. In both cases, the accumulation of a higher stress with time in intrusive rocks is necessary to overcome resistance forces, and thus a higher energy is released during seismic events. Apart from the possible control exerted, at local scale, by rock rheology, we emphasize that all the available data at regional scale, also indicate that the seismic activity is more concentrated in the fold-and-thrust belt of the GC. Here, both the earthquakes with a medium M, as well as those with the largest M, are more frequent than in the SP. This can be explained by the presence of the longest surface traces of the seismogenetic faults in the GC, which should correspond to the widest slip planes. In the SP instead, the surface fault traces are shorter, and thus should be the expression of smaller fault planes capable of producing earthquakes with lower M. This is consistent with the structural surface and subsurface data of Sobornov (1994), which indicate the presence in the Dagestan area of a series of minor reverse recent faults. Moreover, seismogenetic faults traceable at the scale of our study are not present in the central and western part of the SP, thus also explaining the lower amount of earthquakes in the SP.

31

The fact that the recurrence period of earthquakes with MS = 7 in the fold-andthrust belt of the GC is about 100 ys, with respect to the 300 ys of the same earthquake type in the SP, suggests that the deformation level of the SP is still at an embryonic stage. Most of the elastic energy induced by the converging motions between the Arabian and European plate is stored and then released in the core of the GC, and only a fraction does propagate further north. 4.3 Active fault kinematics and geometry Most FMS show reverse or strike-slip kinematics. Reverse fault solutions are closely correlated with the active faults of the GC imaged by field geological data or geophysical prospecting (Agayeva et al., 2009; Kadirov et al., 2012; Babayev et al., 2014; Alizadeh et al., 2016). As suggested by the present-day rates of strain accumulation on the main faults of Caucasus (Reilinger et al., 2006), the main reverse faults are located at the southern and northern borders of the mountain chain. The higher activity of the reverse faults along the southern border is further evidenced by the results of continuous GPS measurements conducted during 19982016, which define active convergence between the Lesser Caucasus/Kura Depression and the GC (Kadirov et al., 2008, 2012, 2015). Based on our results, most reverse faulting occurs in the central and western part of the GC at crustal level (< 30 km), along WNW-ESE-striking planes. Based on field data, geophysical sections and FMS, in this area active faults dip NE (Fig. 14A). In the westernmost part of the GC, active reverse faulting is concentrated along the southern slope of the mountain chain, as suggested also by paleoseismological investigations at some artificial trenches (Rogozhin et al., 2002). FMS with strike-slip kinematics, shallower than 30 km, are mostly located along the southern margin of the eastern GC. These events are characterised by one slip plane parallel to the mountain range, and thus parallel to the faults mapped by field data and geophysical crustal exploration data, as shown in Figure 14B. This strikeslip mechanism can compensate part of the shortening between the Eurasian and the Africa-Arabian plates by lateral escape of tectonic blocks. Lateral escape is a well-known mechanism that occurs at several active mountain belts and intracontinental platforms (e.g. Tapponnier et al., 1982; Burke and Sengör, 1986). In the GC, this mechanism might occur at a more local level, because there is no evidence in the field of very large active strike-slip faults that promote large-scale

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crustal block escape. The only large-scale transcurrent faults that have been proposed to dissect the GC are represented by the NE-striking left-lateral strike-slip fault and the NW-striking right-lateral strike-slip fault located in the northeastern part of the mountain belt (Fig. 1). Some strike-slip FMS, in fact, can be associated with these faults on the basis of the epicenters locations and the presence of a slip plane parallel to the mapped faults. These two strike-slip faults do not appear to be consistent with extrusion tectonics; they seem instead compatible with the orogenic front propagating northward into the foreland basin, if we consider that they are linked together by the WNW-ESE-striking Dagestan reverse blind fault in the northernmost part. The Dagestan thrust dips to the SSW and belongs to a split-apart system accompanied by a series of minor reverse faults dipping NNE that reach the surface (Sobornov, 1994) (Fig. 14B). Our FMS analysis confirms the reverse activity along the faults of this split-apart system, with hypocenters at depths of 5-30 km. Here, the orientation of the slip planes of the FMS is WNW-ESE, consistent with the strike of the Dagestan thrust and its associated minor faults. Other authors suggested that large-scale strike-slip faults cut the GC and separate the weak lithosphere of the western part of the mountain range from the strong lithosphere that characterizes the central and eastern portions of the range (Ruppel et al., 1990; Zoback et al., 2003). Nevertheless, we observe that in areas C and E the dominant strike-slip FMS can coincide with the fault planes parallel to the mountain southern front. In this case, the reverse offsets seen in the field data and geophysical sections (e.g. Tsereteli et al., 2016) could represent a component of the total cumulated net slip, being strike-slip the other component (Fig. 14B). The resulting dominant kinematics is thus of right-lateral transpressional type along the WNW-ESE-striking faults. This is compatible with the eastward tectonic escape of the central-eastern part of the GC, as a consequence of the collision between the structurally thickened crust of the GC and Lesser Caucasus immediately to the west. The eastward escape is supported by the E-W compression that affects the South Caspian basin (Kopp, 1997). This setting might represent the first stage of an incipient process of indentation. Our updated catalogue, which has a larger number of earthquakes with respect to previous investigations, indicate the presence of only a couple of earthquakes with MS > 3.5 below 25 km of depth (down to 50 km) under the western GC, west of 45°E (Fig. 14A). Based on our data, in the eastern CG instead, reverse faulting is more

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diffuse at depths > 30 km, where several earthquakes with MS > 3.5 do occur (Fig. 14B). Mumaldze et al. (2015) suggested that below the central-eastern GC, the deeper earthquakes depict a northeast-dipping zone of hypocenters that reaches the mantle.

Figure 14. Sections across western (A-B) and eastern (C-D) Greater Caucasus, showing events (MS > 3.5) relocated by the present study, and possible traces of the main active faults. Geometry of active faults is based on field data, distribution of hypocenters and geophysical sections of Banks et al. (1998) and of Sobornov (1994) for section A-B and C-D, respectively. Kinematics of active faults is based on the present study. Surface trace of sections are shown in Figure 5. 4.4 Present-day stress field The variations of the principal stress axes as obtained by inverting the FMS presented in this study, indicate some spatial heterogeneity. The stress field consists of a quite stable 1 that remains horizontal, whereas the  and 3 permute each other from a region to another. This permutation might be linked to the heterogeneity of the lithosphere beneath the GC (Yetirmishli et al., 2018). A further explanation is linked to the fact that, moving eastward, the shortening rate across the GC increases

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(Reilinger et al., 1997, 2006). In the central-eastern mountain belt, the higher strain rate may promote a more complex fault activity interesting tectonic blocks of smaller size. This penetrative tectonics is reflected by the more diffuse seismicity here, and by the presence of the aforementioned stress heterogeneity with coexistence at short distance of strike-slip and reverse motions. This complexity is also testified by the presence of seismic fault activity at transverse and orthogonal faults throughout the territory of Azerbaijan (Khain et al., 2005). In spite of these frequent permutations of  and 3, it is important to highlight that the orientation of 1 is instead stable throughout the GC, being mostly NNE-SSW and secondarily NE-SW (Fig. 13). This is consistent with previous results that already showed that the main stress axes are approximately normally-oriented to the mountain chain in the northern GC (Babayev, 2009) and in the western GC (Tsereteli et al., 2016). Zone C is the only main tectonic unit where there is an anticlockwise rotation of 1, where it trends NNW-SSE. This orientation of 1 is compatible with the right-lateral strike-slip motions along faults parallel to the mountain chain, as suggested above. We are more inclined to interpret this stress orientation again as the effect of the collision Lesser-Greater Caucasus that occurs in correspondence of the section between the Georgian block (zone A) and the westernmost part of zone C. Active strike-slip tectonics along faults parallel to the mountain belt in convergence settings have been observed also in several other regions (e.g. Tibaldi and Ferrari, 1992; Pasquaré and Tibaldi, 2003). Finally, from a methodological point of view, Tai-LinTseng et al. (2016) obtained new FMS and stress variations for the southern GC based on the waveforms detected by a new seismic array deployed between 2008 and 2012 for small to moderate earthquakes. They adopted the full-waveform inversion procedure developed by Kao et al. (1998) for regional cases. Their results are in good agreement with our data obtained from older instruments for the same area. The similarity of results obtained by different methodologies for the same tectonic units validates the correctness of estimating FMS by using standard first motion polarity technique. 4.5 Distribution of volcanoes and state of stress In Figure 13 we show all the Quaternary volcanoes of the GC as reported in the most recent catalogue of volcanism distribution by Lebedeva and Vashakidzeb

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(2014). All of them are located in the western and central axial part of the mountain belt. The western group, which includes Elbrus volcano, is composed of a cluster of ten volcanic centers, and two further centers shifted to the northeast. All 12 volcanoes are distributed north of the south-vergent system of active thrust faults that characterizes this part of the GC. Here, the stress field is given by horizontal WNWESE 2 and NNE-SSW 1. The area occupied by Quaternary volcanism is characterized by a low level of seismicity, both in terms of low magnitude events and low frequency. We interpret this as the evidence of magma upwelling in a zone of relatively lower crustal stress. The eastern group is characterized by a larger concentration of volcanic centers with a cluster of tens of volcanoes, two isolated centers to the west, and one to the east, ending with the Kazbek stratovolcano. All of them are distributed north of the zone of collision between the Lesser and the Greater Caucasus, in an area close to the main compressional faults that characterize the southern side of the mountain belt. This area of volcanism is located between the tectonic zone A, characterized by dominant active reverse faulting with a NNW-SSE 1, and tectonic zone C, affected by right-lateral strike-slip to reverse right-lateral faults. The various volcanic centers are located in an area with a high concentration of earthquakes of medium and large size. This suggests that the volcanoes are located on top of a crustal sector characterized by large active compression with a general transpressional type of deformation. This state of stress has been considered, for a long time, unfavourable to magma upwelling due to the presence of a horizontal 1, as reviewed in Tibaldi et al. (2009). Notwithstanding, our research confirms the possibility that magma can reach the surface also in crustal sectors under active large horizontal compression. Although for other compressional belts it has been proposed that magma transport is guided by dilatancy in the core of anticlines above a basal detachment and successive upward expulsion of magma during anticline tightening (e.g. Kruger and Kisters, 2016), our data suggest another mechanism. Based on present knowledge, in the area of Kazbek volcano we can exclude the hypothesis of fold control, because the subvolcanic succession here is characterized by steeply dipping, almost vertical strata. We are more inclined to suggest that magma upwelling occurred along preexisting mechanical discontinuities, in the form of faults and subvertical layering. 5 Conclusions

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In the GC, a dominant NNE-SSW-oriented compressional stress field controls the overall occurrence of earthquakes. A total of 366 focal mechanism solutions, 239 of which are new and the others have been selected from previous catalogues, indicate that most seismic events are represented by reverse faulting or strike-slip faulting; both kinematics coexist in most of the various main tectonic zones of the mountain belt, suggesting a local permutation of the 2 and 3 axes, whereas the 1 remains horizontal. An analysis of kinematics versus earthquake magnitude indicates that the larger events have a reverse faulting kinematics, thus smaller events might be driven by local stress fields where strike-slip motions might also occur. This indicates a split of the main fault systems into smaller segments at some places, allowing low to medium magnitude earthquakes to occur independently from the dominating, largescale stress field. Moreover, at depths < 30 km, reverse kinematics dominate along the northern and southwestern flanks of the GC, whereas strike-slip kinematics is dominant along the southeastern flank. This is here interpreted as the possible effect of a component of eastward escape of the central-eastern part of the mountain range along WNWESE strike-slip right-lateral faults and transpressional faults, following the collision between the structurally thickened crust of the central Greater and Lesser Caucasus. At depths > 30 km, seismicity tends to disappear below the western GC, whereas below the central-eastern mountain belt earthquakes occur also in the mantle with dominant reverse faulting. Our data confirm that in the western GC, shortening takes place by pure reverse faulting along slip planes parallel to the mountain belt, with a southward vergence of active structures. In the eastern GC, at depths < 30 km, shortening seems to be more absorbed by the structures located along the northern side of the mountain belt, where FMS with reverse slips are present. Here, a main north-vergent active structure is represented by the Dagestan blind thrust, which allows propagation of shortening northward creating a main salient. A component of shortening is present also along the southern border of the eastern GC with southward vergence. The GC is also a clear example of association between active volcanism and contemporaneous tectonic compression with horizontal  and . Although this setting has been classically considered unfavourable to magma eruption, we suggest

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that a sufficient magma overpressure can exceed the intermediate compressive stress along pre-existing suitably-oriented mechanical discontinuities such as faults or layering.

Acknowledgments We acknowledge the useful review and suggestions of Randell Stephenson on a previous version of the manuscript. Federica Lanza is acknowledged for her revision of the English grammar. This study has been conducted in the framework of: NATO project SfP G4934 "Georgia Hydropower Security", International Lithosphere Program - Task Force II, European Space Agency project n. 32309 “Active tectonics and seismic hazard of southwest Caucasus by remotely-sensed and seismological data (Leader A. Tibaldi), and project 216758 of the Shota Rustaveli National Science Foundation. AT coordinated the research and wrote the discussion, OV prepared chapters 3.1 and 3.2, TM provided catalogue of NC and correlation between K and ML. AB carried out the stress inversion. NT calculated FMS of Georgia and contributed to write data chapters.

GB, FK, GY and SK prepared the seismic

catalogue and FMS of Azerbaijan. FB and ER contributed to write the data chapters and some of the related figures. Supplementary documents Table 1s. Fault plane solution parameters of the studied events. The table reports date, origin time, strike, dip and slip for first (N1_Str1, N1_dip1, N1_slip1) and second (N2_Str2, N2_dip2, N2_slip2) nodal planes, azimuth and plunge of P and T axes, FPS category (grade), number of polarities (p_n), quality factor for earthquakes Q.

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Graphical abstract

Highlights

Different earthquake catalogues for the Greater Caucasus have been homogenized 239 new focal mechanisms and related stress inversion show active fault kinematics Reverse faulting dominates in the western and NE Caucasus, strike-slip in the SE Lesser-Greater Caucasus collision induces stress rotation and E-ward escape tectonics

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Author agreement

On behalf of all coauthors, I declare that this submission implies the consent of all the participating authors.

The contribution of each author is declared in the Acknowledgments section; all authors have materially participated in the research and/or article preparation.

I also declare that all authors have approved the final submitted article.

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Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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